The present invention relates to an optical storage apparatus for recording and reproducing information by using a laser beam and a recording and reproducing method of an optical storage medium. More particularly, the invention relates to an optical storage apparatus for recording and reproducing data at a density smaller than a beam diameter known as a magnetically induced super resolution and a recording and reproducing method of an optical storage medium.
In recent years, an optical disk is spotlighted as an external storage medium of a computer. According to the optical disk, by forming magnetic recording pits on the submicron order onto a medium by using laser beam, a recording capacity can be remarkably increased as compared with that of a floppy disk or a hard disk serving as a conventional external storage medium. Further, in a magnetooptic disk as a perpendicular magnetic storage medium using a material of the rare earth--transition metal system, information is rewritable and a development in future is expected more and more.
For example, the optical disk has a storage capacity of 540 MB or 640 MB per side of 3.5 inches. This means that a storage capacity of one 3.5-inch floppy disk is equal to about 1 MB and one optical disk has a storage capacity of 540 or 640 floppy disks. As mentioned above, the optical disk is a rewritable storage medium having a very high recording density. In order to prepare for a coming multimedia age, however, it is necessary to further increase the recording density of the optical disk to a value higher than the present one. In order to increase the recording density, more pits have to be recorded on the medium. For this purpose, it is necessary to further reduce the pit size to a value smaller than the present pit size and to narrow the interval between the pits. In case of increasing the recording density by such a method, it is necessary to further shorten a wavelength of laser beam to a value shorter than the present wavelength of 670 nm. When a practical use is considered, however, the pit size has to be reduced at the present wavelength of 670 nm. In this case, with respect to the recording, a pit smaller than the beam diameter can be formed by controlling a power of the laser beam. With respect to the reproduction, however, when the pit smaller than the beam diameter is reproduced, a crosstalk with the adjacent pit increases and, in the worst case, the adjacent pit also enters the reproducing beam. It is, therefore, very difficult to form such a small pit when the practicality is considered.
As a method of reproducing the pit smaller than the beam diameter by the present wavelength of 670 nm, there is a magnetooptic recording and reproducing method represented by JP-A-3-93058. Such a method is known as a recording and reproducing method by the MSR (Magnetically induced Super Resolution). The method has two methods of an FAD (Front Aperture Detection) method and an RAD (Rear Aperture Detection) method.
According to the FAD method, as shown in FIGS. 1A and 1B, a storage medium is divided into a recording layer 220 and a reproducing layer 216 and information is reproduced by applying a reproducing magnetic field Hr to the recording medium in a state where a laser spot 222 of a read beam is irradiated thereto. In this instance, with respect to a portion of a recording pit, a magnetic coupling of a switching layer 218 formed in a boundary between the reproducing layer 216 and the recording layer 220 is released depending on a temperature distribution of the medium heating by the laser spot 222. The reproduction layer 216 is influenced by the reproducing magnetic field Hr and becomes a mask. On the contrary, with respect to a portion of the next recording pit, the magnetic coupling of the switching layer 218 is kept and the portion becomes an opening 224. Consequently, only a pit 230 of the opening 224 can be read without being influenced by a neighboring pit 226 as in case of the laser spot 222.
On the other hand, according to the RAD method, as shown in FIGS. 2A and 2B, an initialization for aligning the magnetizing direction of the reproducing layer 216 into a predetermined direction is executed by using an initializing magnet 232 and the reading operation is performed by slightly increasing a reproducing laser power at the time of reproduction. Upon reading, a mask 236 in which initial magnetization information remains and an opening 238 in which the initial magnetization information is erased and to which magnetization information of the recording layer 220 is transferred are formed in the reproducing layer 216 depending on the temperature distribution of the medium heating by a laser spot 234 of the read beam. The magnetization information of the recording layer 220 transferred to the reproducing layer 216 is converted into an optical signal by a magnetooptic effect (Kerr effect or Faraday effect), thereby reproducing data. In this instance, on the contrary to a pit 228 in the recording layer 220 which is being read at present, information is not transferred to the pit 230 in the recording layer 220 to be read out next because the mask 236 is formed by the initial magnetization information in the reproducing layer 216. Consequently, even if the recording pit is smaller than the laser spot 234, a crosstalk does not occur and a pit smaller than the beam diameter can be reproduced. Further, by using the magnetically induced super resolution, since the region of the recording layer 220 except for a reproducing portion is masked by the initialized reproducing layer 216, a pit interference from the adjacent pit does not occur and the pit interval can be further narrowed. Since a crosstalk from the neighboring track can be also suppressed, the track pitch can be also decreased and the density can be increased even if the present wavelength of 780 nm is used.
The conventional optical disk apparatus using the magnetically induced super resolution has, however, a problem such that if the intensity of the reproducing magnetic field which is used at the time of reproduction is not strictly controlled, a proper reproducing operation cannot be executed. The reason is as follows. For example, when the reproducing magnetic field Hr is too weak in the FAD method of FIGS. 1A and 1B, the forming range of the mask 226 in FIG. 1B by the magnetization of the reproducing layer 216 decreases, so that the pit 228 is not masked and a crosstalk occurs. When the reproducing magnetic field is too strong, the forming range of the mask 226 is widened, the pit 230 is also partially masked, a reproducing level decreases, and an error occurs. The reproducing magnetic field Hr simultaneously acts on the recording layer 220 as well and the recording data may be erased.
When the initializing magnetic field is too weak in the RAD method of FIGS. 2A and 2B, an erasing range by the beam heating of the initial magnetization of the reproducing layer 216 is widened and the forming range of the mask portion decreases, the pit 230 in FIG. 2B is not masked, and a crosstalk is caused. When the initializing magnetic field is too strong, the erasing range by the beam heating of the initializing magnetic field of the reproducing layer 216 is narrowed, the forming range of the mask 236 is widened, the pit 228 is also partially masked, the reproducing level decreases, and an error occurs. Simultaneously, when the initializing magnetic field is too strong, it also acts on the recording layer 220 and the recording data may be erased. It is insufficient for such a phenomenon even if the reproducing magnetic field and the initializing magnetic field are merely adjusted and this phenomenon also depends on the environment temperature in the apparatus which determines the temperature of the storage medium. That is, when the environment temperature in the apparatus changes to the low temperature side, hysteresis characteristics of the reproducing layer become fat. In order to obtain the same magnetizing characteristics (magnetic flux density), the reproducing magnetic field has to be made strong. On the contrary, when the environment temperature changes to the high temperature side, the hysteresis characteristics of the reproducing layer become thin, so that the reproducing magnetic field has to be weakened in order to obtain the same magnetizing characteristics.